ATF1 and sarcoma: To examine whether Tppp3-expressing cells can give rise to EWS/ATF1-induced sarcomas, we established compound transgenic mice that harbor Tppp3-CreERT2 allele together with Rosa26-loxP-stop-loxP (LSL)-rtTA3 allele and Col1a1:: tetO-EWS/ATF1, in which EWS/ATF1 can be induced with Dox exclusively in Tppp3-expressing cells after tamoxifen treatment (Fig. 6a).